1. Field of the Invention
This invention relates generally to improvements in transient noise filters and, more particularly, but not by way of limitation, to a transient noise filter in which portions of an input signal distorted by transient noise pulses are replaced by an estimate of the input signal upon the detection of a transient noise pulse having predetermined characteristics.
2. Description of the Prior Art
Prior art noise filtration systems may be generally classified as frequency fitering, dynamic volume control, or clipping. Each of these types of circuits has disadvantages, however, when applied to the removal of transient noise pulses in such typical applications as the record playback or radio reception of audio signals such as music. Due to these limitations, none of these systems are fully satisfactory in such applications. Since such applications are perhaps the most demanding on noise filtration systems, the following discussion will be directed to the problems associated with the elimination from a program signal, such typical transient noise pulses as those caused by dust or scratches on the surface of a phonograph record or by radio static.
Frequency filtration systems remove predetermined frequency ranges under the assumption that the eliminated frequencies contain relatively more noise and less signal than the nonfiltered frequencies. While this assumption may be valid in general as to those frequencies filtered, these systems do not even attempt to remove the components of the noise lying within the non-filtered frequencies nor do they attempt to salvage any program signal from the filtered frequencies. In effect, these systems muffle the noise and also part of the program.
Dynamic volume control systems modify system gain as a function of the signal level thus passing fully only the louder, more noise resistant portions of the signal and suppressing the weaker portions. An extreme example of this system is utilized in the "squelch" circuit of radios. Although there will be the illusion of suppressing faint noise, the instantaneous signal to noise ratio is not improved, and there may be the illusion of actually amplifying noise transients which are louder than the program signal. In some of these systems, the volume change is greater at the higher frequencies, thus combining a type of frequency filtering with the dynamic volume control. However, the unnatural sound produced by this system must be overcome by boosting the volume of weaker passages of the program signal at the time it is being recorded, which amplifies during playback, the normal flutter in volume and frequency response variations caused by system imperfections.
Clipping circuits prevent the combined program signal plus noise from exceeding predetermined limits. Thus, as long as the magnitude of the signal remains within the limits, it is passed unaffected but the combined signal is clipped to the predetermined level when it contains noise components which are of sufficient magnitude to force the total magnitude beyond the limits. However, the program signal is similarly clipped when its magnitude exceeds the predetermined limits while noise up to the limits is passed unaffected. In some systems, the limits are dynamically varied as a function of signal magnitude. Sometimes, the output is set to zero or some other signal when the clipping limits are exceeded. However, since the limits should never be set lower than the current signal level waveform peak and in fact should allow a margin of safety for reasonable transients in the program signal, these systems in effect act only on noise which is greater in magnitude than the program signal and thus are of use only for controlling gross noise.
One such clipping circuit is disclosed in U.S. Pat. No. 3,700,812 issued to James C. Springett on Oct. 24, 1972. The Springett circuit utilizes either digital or analog devices to compare the amplitude of the envelope of the composite signal, as measured over a predetermined time period, to the instantaneous amplitude of the signal at the center of the predetermined time period, and, when the envelope amplitude is exceeded by the instantaneous amplitude, substitutes a pulse whose amplitude is either that of the envelope or a signal segment adjacent to the noise segment. This system is essentially a clipping circuit which varies the clipping level as a function of signal volume. Its ability to discriminate transient noise pulses is limited due to its simplistic approach. Further, the substitute signal represents at best a gross approximation of the desired signal.
Although combinations of the above described systems have been developed which have somewhat improved operating characteristics, the primary disadvantage remains that not all of the components of the noise pulse are effectively filtered or removed, and not all of the signal is passed. The result is still a discernable noise coupled with a loss of signal quality.
An effective transient noise filter should be capable of detecting noise pulses through the examination of more pulse characteristics than instantaneous amplitude and should substitute for the noise pulses a signal representing a good estimate of the noise distorted portion of the program signal.